Hi, I keep a Rouen duck so I guess that considering that they are mallard subspecies I should know about them, the pros are that if you get them as ducklings they grow up to be really tame and they love attention, not to mention that they follow you around everywhere.
The cons are that they are sooooooooooo stubborn, like pigs except not so bad (own a pig).
Sometimes they can be a bit rough when giving out kisses because they get over enthusiastic, and, like all ducks they are messy, but lovable, I recon that if you have the space (I keep them in a pen roughly 13mx10m with a pool about 7mx4m) then mallards are wonderful!